A wireless communication system in which an Orthogonal Frequency Division Multiplexing (hereinafter referred to as “OFDM”) transmission system is adopted has been known. The OFDM is a kind of multi-carrier modulation system and has, compared with a conventional single-carrier modulation system, higher resistance to multi-path fading caused when channel is intricate due to obstacles.
However, even though an OFDM signal is used, when a desired signal to noise power ratio (hereinafter referred to as “SNR”) is not obtained due to lower received power of sub-carriers at specific frequencies caused by multi-path fading, as shown in FIG. 15, part of data cannot be demodulated, causing reduced transmission capacity as a system.
To solve such a problem, a technique of applying Multilevel Transmit Power Control (hereinafter referred to as “MTPC”) has been proposed in which an adaptive modulation scheme is applied by which sub-carriers whose attenuation of received power is significant due to multi-path fading are transmitted using a small multilevel modulation scheme and sub-carriers whose attenuation of received power is small are transmitted using a high multilevel modulation scheme, and transmission power of sub-carriers that transmit data is adjusted so that a desired SNR is obtained. This MTPC system is a system that is gaining attention as a countermeasure against multi-path fading from the viewpoint of limiting a maximum value of transmission power and the like and using sub-carriers efficiently.
FIG. 17 is a diagram showing a configuration example of a frame format of a wireless communication system in which the OFDM/MTPC system is adopted. This frame format is used when, for example, establishing a downlink from a base station apparatus to a mobile station apparatus. As shown in FIG. 17, a transmission frame (communication frame) 201 is composed of 10 slots 202-1 to 202-10. Each of the slots 202-1 to 202-10 is primarily composed of two parts; a synchronization/control data part 203 and a user data part 204.
The synchronization/control data part 203 includes a Channel Estimation word 205 (hereinafter referred to as “CE”) known to a receiving side and is used for estimating channels and modulation level information 206 (hereinafter referred to as “MLI”) to notify the receiving side of a modulation level of each sub-carrier that transmits user data. These define the modulation level of each sub-carrier and transmission power of each sub-carrier, and are features of the OFDM/MTPC system. Here, the MLI is updated for each communication frame.
When transmitting a signal in the frame format shown in FIG. 17, the synchronization/control data part 203 is transmitted using the OFDM system. That is, the same modulation level is applied to all sub-carriers with the same transmission power.
The user data part 204 is transmitted using the MTPC system. That is, each sub-carrier is transmitted by a modulation levelwith adifferent multilevelmodulation scheme andtransmission power is controlled for each sub-carrier. More specifically, the following is done:    (1) The modulation level for each sub-carrier is one designated by MLI of the synchronization/control data part.    (2) Transmission power of each sub-carrier is adjusted depending on quality of channels so that a desired reception SNR is obtained for each sub-carrier on the receiving side.    (3) A sub-carrier whose channel is of extremely low quality may be made a carrier hole by providing no transmission power.
Since fluctuation velocity of a channel with respect to a frame length is slow in general communication, transmission power and the modulation level do not need to be changed within the same frame. Consequently, there is no need to change transmission power and the modulation level within the same communication frame. Thus, MLI is all the same within the same communication frame.
Next, a configuration example of a mobile station apparatus applied to an OFDM/MTPC communication system will be described. As shown in FIG. 18, a mobile station apparatus 208 has a receiving circuit 209 and a transmitting circuit 210. An RF signal received by a receiving antenna 211 is down-converted by an RF converter 212 and input into the receiving circuit 209. An output signal of the RF converter 212 input into the receiving circuit 209 is input into an analog/digital conversion circuit 213 to convert the signal from an analog signal into a digital signal. A digital signal output by the analog/digital conversion circuit 213 is input into a demultiplexer 214 to demultiplex and output the signal to a CE part 205, an MLI symbol part 206, and a user data symbol part 204 in accordance with a slot configuration shown in FIG. 17.
A Fourier transformation circuit (FFT circuit) 215-1 performs a Fourier transformation of an output signal of the demultiplexer 214 to reproduce a received CE. A channel estimation circuit 216 compares a received CE input from the Fourier transformation circuit 215-1 and a reference CE to estimate channel characteristics.
A Fourier transformation circuit (FFT) 215-2 performs a Fourier transformation of an output signal of the demultiplexer 214 to reproduce a received MLI symbol. A channel compensation circuit 217 makes channel compensation for a reproduced received MLI symbol based on an estimation result of the channel estimation circuit 216. A symbol demodulation circuit 218 demodulates MLI from the received MLI symbol for which channel compensation has been made by the channel compensation circuit 217. An error detection circuit 219 detects errors from an output signal of the symbol demodulation circuit 218 using error detecting code and the like.
A demodulation level designation circuit 220 designates a demodulation level of each sub-carrier of user data based on the demodulated MLI.
A Fourier transformation circuit (FFT) 215-3 performs a Fourier transformation of an output signal of the demultiplexer 214 to reproduce a received user data. A channel compensation circuit 221 makes channel compensation for a reproduced received user data symbol based on an estimation result of the channel estimation circuit 216. A symbol demodulation circuit 222 demodulates the received user data symbol for which channel compensation has been made by the channel compensation circuit 221 by a demodulation level of a user data symbol part of each sub-carrier designated by the demodulation level designation circuit 220. A decoder circuit 223 performs error correction and decompression processing of encoded user data demodulated by the symbol demodulation circuit 222 to decode user data.
In the receiving circuit 209 shown in FIG. 18, components for demodulating CE, MLI, and user data can be summarized as shown below:    (1) A CE demodulation part composed of the FFT circuit 215-1    (2) An MLI demodulation part 224 composed of the FFT circuit 215-2, the channel compensation circuit 217, the symbol demodulation circuit 218, and the error detection circuit 219    (3) A user data demodulation part 225 composed of the FFT circuit 215-3, the channel compensation circuit 221, the symbol demodulation circuit 222, and the decoder circuit 223
Also, transmission data (user data) is input into the transmitting circuit 210. In the transmitting circuit 210, for example, coding processing, modulation processing, and processing to feedback a channel estimation result signal input from the channel estimation circuit 216 to a base station as information data are performed with respect to the transmission data. Then, the transmission data undergoes digital/analog conversion, and is up-converted into an RF signal by an RF converter 226 and transmitted by a transmitting antenna 227.
Next, a configuration example of a base station apparatus applied to an OFDM/MTPC communication system will be described. As shown in FIG. 19, a base station apparatus 230 has a transmitting circuit 231 and a receiving circuit 232. In the transmitting circuit 231, a modulation level/transmission power designation circuit 233 determines, based on a channel estimation result signal acquired as received data by the receiving circuit 232, transmission power of each sub-carrier for transmitting user data (transmission data) and the modulation level of each sub-carrier for transmitting user data.
An encoder circuit 234 performs processing such as compression coding of user data (transmission data) and addition of error correction code, and a symbol modulation circuit 235 modulates, based on the modulation level of each sub-carrier determined by the modulation level/transmission power designation circuit 233, user data encoded by the encoder circuit 234. A transmission power control circuit 236 regulates an output signal from the symbol modulation circuit 235 to a value determined by the modulation level/transmission power designation circuit 233 for each sub-carrier, and an IFFT circuit 237 performs an inverse Fourier transformation of an output signal of the transmission power control circuit 236 for output.
An MLI generating circuit 238 generates MLI based on the modulation level of each sub-carrier for transmitting user data determined by the modulation level/transmission power designation circuit 233. A symbol modulation circuit 239 modulates MLI generated by the MLI generating circuit 238. An IFFT circuit 240 performs an inverse Fourier transformation of an output signal of the symbol modulation circuit 239 for output.
A CE generating circuit 241 generates a CE and an IFFT circuit 242 performs an inverse Fourier transformation of a CE generated by the CE generating circuit 241 for output.
A multiplexer 243 multiplexes output signals of three IFFT circuits (237, 240, and 242) to match the slot configuration shown in FIG. 17. A digital/analog conversion circuit 244 converts an output of the multiplexer 243 from a digital signal into an analog signal. An analog signal output by the digital/analog conversion circuit 244 is up-converted into an RF signal by an RF converter 245 and transmitted by a transmitting antenna 246.
In the transmitting circuit 231 shown in FIG. 19, components for modulating CE, MLI, and user data can be summarized as shown below:    (1) A CE modulation part 247 composed of the CE generating circuit 241 and the IFFT circuit 242    (2) An MLI modulation part 248 composed of the MLI generating circuit 238, the symbol modulation circuit 239, and the IFFT circuit 240    (3) A user data modulation part 249 composed of the encoder circuit 234, the symbol modulation circuit 235, the transmission power control circuit 236, and the IFFT circuit 237
An RF signal received by a receiving antenna 250 is down-converted by an RF converter 251 and input into the receiving circuit 232. In the receiving circuit 232, for example, analog/digital conversion processing, demultiplexing processing into various signals, and various demodulation processing are performed to output received data (user data).    Non-patent document 1: The Institute of Electronics, Information and Communication Engineers RCS2002-239 “Study on interference reducing technology in a one-cell repetitive OFDM/TDMA system using a sub-carrier adaptive modulation system”